A surgical catheter 1 for real-time mapping of the electrical activity of the heart. The catheter 1 has a shaft 1b and a distal tip 1a, which is inserted into the atria 2 of a patient. A microwave emitting element 3 transmits microwave radiation, and a receiving sensor measures the reflected emissions. The sensor may be integrated into the same catheter tip, on a second tip, or on part of the shaft. Alternatively the sensor may be on a second, separate catheter. The measurements can be used to create a temporal map of the electrical activity, processed by dielectric spectroscopy. The emitted radiation is tuned to the hyper-resonance or resonance frequency of hydrated potassium ions, and the reflected emissions measure the concentration of hydrated potassium ions within a particular volume.
